When Google Calendar changed the favicon, many people wondered why it only shows 31. "Does it mean that starting today, every day will be the 31st ? Serously, favicon should be changed everyday to match the date," suggested a Google Calendar user.
Google listened to the feedback and the favicon will now change every day. "When you look at the Google Calendar icon at the top of your browser window, it will no longer always display 31 but will instead change to reflect the current day of the month. Today's date is now always a short glance away."
Google Calendar is the first Google service that has a dynamic favicon, but you can change Gmail's favicon to show the number of unread messages.